The workaround is to default these attributes to NULL instead. To do this:
Find the attribute you are trying to default in Schema Designer. (For example, a Request's Impact would be found under cr (Request) -> impact (Impact))
After selecting the attribute click on the Advanced tab on the right pane.
Under Site-Defined Triggers put: "ON_NEW DEFAULT "NULL";" without the outer quotes.
Repeat for all attributes you want to set your own default for.
Click File -> Save and Publish
Stop Service Desk Manager and run pdm_publish.
Start Service Desk Manager and you should see your default values take place.